The present invention relates to linear beam electron devices, and more particularly, to an electron gun that provides multiple convergent electron beamlets suitable for use in a multiple beam klystron using confined flow magnetic focusing.
Linear beam electron devices are used in sophisticated communication and radar systems that require amplification of a radio frequency (RF) or microwave electromagnetic signal. A conventional klystron is an example of a linear beam electron device used as a microwave amplifier. In a klystron, an electron beam is formed by applying a voltage. potential between a cathode emitting electrons and an anode accelerating these emitted electrons such that the cathode is at a more negative voltage with respect to the anode. The electrons originating at the cathode of an electron gun are thereafter caused to propagate through a drift tube, also called a beam tunnel, comprising an equipotential surface, thereby eliminating the accelerating force of the applied DC voltage. The drift tube includes a number of gaps that define resonant cavities of the klystron. The electron beam is velocity modulated by an RF input signal introduced into the first resonant cavity. The velocity modulation of the electron beam results in electron bunching due to electrons that have had their velocity increased gradually overtaking those that have been slowed. Velocity modulation in the gain section of the tube leads to bunching, i.e. the transformation of the electron beam from continuously flowing charges to discrete clumps of charges moving at the velocity imparted by the beam voltage. The beam bunches arrive at the bunching cavity, sometimes called the penultimate cavity, where they induce a fairly high RF potential. This potential acts back on the beam, and serves to tighten the bunch. When the bunches arrive at the output cavity they encounter an even higher rf potential, comparable to the beam voltage, which decelerates them and causes them to give up their kinetic energy. This is converted to electromagnetic energy and is conducted to a load. The tighter the bunching, the higher the efficiency. However, a high degree of space charge concentration interferes with the bunching process and the efficiency. Other things being equal, the higher the perveance of a klystron, the lower the efficiency.
The effect of perveance on the gain of a klystron is different. Although the gain is affected by space charge, it is a stronger function of the total current, which is proportional to the perveance. This suggest that if a beam cross-section were made larger, so that the current density and space charge are reduced, both gain and efficiency would benefit. However, such is not the case because a large beam requires a large drift tube, and the electric fields which couple the beam to the circuit fall off across the beam, leading to poor coupling and a drop in both gain and efficiency. A small beam is therefore necessary, but if the power output required is high, the voltage, rather than the current in the beam must be increased for reasonable efficiency.
Bandwidth is inversely proportional to the loaded Qs of the klystron cavities. In the gain section of the tube, where cavities are stagger-tuned, the cavity Qs are loaded by the beam. The higher the current, the higher the loading, and consequently the lower the Q. It does not matter if a single beam or several beams are traversing the cavity. The output cavity, in particular, must by itself have a bandwidth at least equal to the desired bandwidth of the klystron. For the output cavity to produce good efficiency, this bandwidth becomes proportional to the beam conductance. However this leads to higher perveances, and hence lower efficiency. Consequently, in a single beam klstron the efficiency/bandwidth product is approximately constant.
Given the preceding relationships, the advantage of the multiple beam klystron provides is clear. The current is divided into several beams, each with a low space charge, so that it can be bunched tightly in a small drift tube with good coupling coefficient, and hence high efficiency. The gain-bandwidth product is not constant, but increases with the addition of beams. For the same power and gain, the multiple beam klystron is shorter than a conventional klystron.
Despite the potential advantages of multiple beam klystrons, such devices have only been adapted for certain low power or low frequency applications in which a convergent electron beam is not necessary. In these nonconvergent devices, electron beam focusing is provided by immersing the electron gun and drift tubes in a strong magnetic field which guides the electrons along the magnetic flux lines to the drift tubes. In a nonconvergent electron gun, the diameter of the emitting surface is the same as the electron beam that propagates through the RF device. The nonconvergent electron beams of this class of device have limited current density, which prevent them from developing more power at higher frequencies. The amount of current that can be emitted from the cathode is dependent on the size of the emitting surface and the maximum electron emission density that can be provided by the surface. Maximum electron emission densities from typical cathodes operating in the space charge limited regime are on the order of 10-20 amps/cm{circumflex over ( )}2.
In a convergent electron gun, the cathode diameter exceeds the diameter of the final electron beam, which means that more current can be provided. The current gain is proportional to the area compression factor of the gun, which is the ratio of the cathode area to the cross sectional area of the final electron beam. Typical compression factors are 5-20.
Electron beams used for linear RF devices typically employ one of two types of magnetic focusing, which act in addition to the initial electrostatic focusing of a Pierce electron gun, whereby a stream of emitted electrons is initially focused to a region of minimum beam diameter. The first type of magnetic focusing is Brillouin focusing, where the magnitude of the magnetic field in the circuit section of the device precisely balances the space charge repulsion forces within the static beam. An embodiment of such a device is shown in FIG. 1. Electrostatic focusing is used to guide the electron beam from the cathode emitting surface to a point within the anode beam tunnel. A minimum diameter is achieved, and if a counteracting magnetic field were not applied, the beam would begin to diverge due to space charge forces. In Brillouin magnetically focused devices, an axial magnetic field is imposed at the location of the minimum diameter that balances the space charge forces and facilitates transport of the beam through the device.
Unfortunately, the balance between the space charge force tending to expand the beam and the magnetic force tending to confine the beam is no longer equal when electrostatic bunching of electrons occurs, as is required to transform beam power into RF power. Consequently, the beam will expand in regions of high electron density, eventually resulting in impact of electrons with the walls of the beam tunnel. This can result in destruction of the device unless the power deposited is limited. Therefore, Brillouin focused devices are limited in the average RF power and pulse lengths that can be generated.
The alternative is to use convergent, confined flow focusing, as shown in FIG. 2. With confined flow focusing, the magnetic field encompasses the cathode regions of the device where the electron beam is generated. A combination of magnetic and electrostatic focusing is used to guide the electron beam from the cathode into the beam tunnel. With confined flow focusing, the magnetic field can be higher than is required for balancing the space charge forces in the static beam. In typical devices, the magnetic field is 2-3 times the Brillouin value. With confined flow focusing, the convergent electron beam will be contained as it traverses the beam tunnel, even in the presence of electron bunching as used to generate RF power. Consequently, confined flow focused devices are capable of high average power operation.
In typical single beam devices, the magnetic field is generated from a solenoid or permanent magnet symmetrically located with respect to the electron beam, which produces a magnetic field that is radially symmetric about the electron beam, which is typically located on the main axis of the device. This radially symmetric field is necessary for the electron beam to follow its non-divergent axial path. The magnitude and shape of the field in the cathode-anode region is controlled using an iron enclosure around the main solenoid or permanent magnet with an aperture through end plates perpendicular to the device axis, allowing field penetration into the cathode-anode region. Auxiliary coils or permanent magnets may also be used in the cathode-anode region to control the shape and magnitude of the field.
While this works well for single beam devices having a beam tunnel symmetrically located with respect to the magnetic field axis, problems occur for electron guns where the cathode-anode region is radially displaced from the device axis. A radial gradient, or shear, in the magnetic field in the cathode-anode region distorts the magnetic focusing, preventing operation of the device. In order to realize a multiple beam device, it is necessary for most cathode-anode structures to be radially displaced from the device axis.
In light of these limitations, the need for a high power, multiple beam klystrons with confined flow focusing for use with high frequency RF sources is clear.
A device described by Symons [U.S. Pat. No. 5,932,972] provides for a convergent multiple beam gun having a single cathode, a first plurality of conductive grids, a second plurality of drift tubes further containing resonant gaps, and an anode. The first plurality of conductive grids are spaced between the cathode and drift tubes, and contain apertures in locations such that electron beamlets are formed and defined by electrons traveling from the cathode, through the apertures in each of the grids, and into the drift tubes. Each of the grids has these apertures in substantial registration with each other and with respective openings of the plurality of drift tubes.
Symons relies on a plurality of grids to shape the electric potentials to focus the individual beamlets into the respective drift tunnels. In one embodiment of the invention, four separate grids are required to provide the necessary electric field configuration. Ceramic insulators providing a portion of the vacuum envelope of the device must electrically isolate each grid. In addition, a separate voltage is required for each grid.
The device described by Symons does not provide for confined flow focusing, as it can be seen that no magnetic focusing field is applied, and beam focusing is performed entirely by electrostatic potentials applied to the many grids. Consequently, the beam will not be fully confined in the presence of space charge bunching, limiting the average and peak power capability of the device. Further, the device described by Symons applies only to fundamental mode cavities, which limits the frequency at which this technique can be applied.
As the RF frequency increases, the available space for multiple beams through a fundamental mode cavity decreases in proportion to the increase in frequency. Consequently, the number of beams that can propagate through a fundamental mode cavity becomes limited by mechanical and thermal constraints. An alternative is to use a ring resonator circuit as described by Bohlen (U.S. Pat. No. 4,508,992). With a ring resonator circuit, the number of beamlets is not strictly limited by frequency considerations. Bohlen describes a microwave amplifier having an annular cathode, an annular ring resonator for the introduction of RF energy, an annular ring resonator for the removal of RF energy, and an annular collector, all of which are operating in the presence of a magnetic field. This structure enables reduced current densities and the application and collection of RF energy over a large physical area. A disadvantage of this structure is that the annular beam tunnels can allow transmission of higher order cavity modes back toward the electron gun. These modes can lead to undesired bunching of the electron beam and prevent operation at the desired frequency and power. Consequently, the gain of this device is limited to less than 25, and the output power level is limited to a few megawatts.
A multiple beam device using periodic permanent magnet focusing was described by Caryotakis et al (European patent WO 97/38436). This device uses periodic permanent magnet (ppm) focusing. PPM focusing uses an array of permanent magnets with alternating magnetic orientations to produce a focusing magnetic field. The focusing field produced by PPM focusing is axial, as in solenoidal focusing, but alternates direction, unlike solenoidal focusing. PPM focusing has been used for years for beam focusing in traveling wave tubes. The focusing described by Caryotakis only applies to beam confinement within the body or circuit section of the device and is not applicable to the electron gun region. Further it requires a series of cylindrical permanent magnets around each individual beam tunnel. Since these magnets can not tolerate high temperatures, they must be applied after construction of the vacuum envelope of the rf device. High power operation of rf devices requires processing in ovens operated at 400-500 degrees C. in order to obtain sufficient vacuum for operation. Consequently, each beam tunnel must contain its own individual vacuum envelope to provide access for the PPM magnets.
Since the device proposed by Caryotakis does not address the magnetic focusing in the electron gun, the present invention could be adapted to work in conjunction with the device described by Caryotakis.
In view of the limitations of the prior art, the present invention provides for an RF device having convergent multiple beams for use in high frequency, high power RF generators, such as multiple beam klystrons or inductive output tubes (IOT). This device has a plurality of drift tubes for the transport of multiple convergent beamlets in a rectilinear flow. Each drift tube carries an electron beam formed by an individual electron gun, and a plurality of these electron guns is arranged in a circular ring, with each electron gun providing a beam for use by an associated drift tube. Each electron gun has a cathode, an electrostatic focusing electrode and anode structure. The path of the confined flow of electrons from each electron gun through the drift tubes of the device forms a beam tunnel, and each separate gun has its own separate beam tunnel. Gaps between drift tubes form resonant cavities for the introduction and removal of RF power and for increased bunching of the electron beam. The RF power introduced into an input port of the device operates on each individual beamlet traveling through each individual beam tunnel, and RF power extracted at the output port is summed by the RF output structure. In the context of the present device, a high power composite electron beam is formed which comprises the contribution of each individual beamlet, so the output power of the device is limited only by the number of beamlets that are contributing to the RF output port. While the-beamlets formed by each electron gun travel through separate beam tunnels, the anode structure and cathode structure for each gun may be separate, or it may be shared.
In one embodiment of the invention, the beam tunnels for each electron beam include drift tubes having a first resonant cavity defined by a first gap provided in the plurality of drift tubes, and a second resonant cavity defined by a second gap provided in the plurality of drift tubes. An electromagnetic signal is coupled into an RF input port to the first resonant cavity, which velocity modulates the beamlets traveling in the plurality of drift tubes. The velocity modulated beamlets then induce an electromagnetic signal into the second resonant cavity, which may then be extracted from the device RF output port as a high power microwave signal. Other resonant cavities may also be applied between the first and final resonant cavity to increase the gain, bandwidth and efficieny of the device. A collector is disposed at respective ends of the plurality of drift tubes, which collects the remaining energy of the beamlets after passing across the various cavities. A magnetic field oriented coaxially to the beam tunnel is furnished to provide confined flow of the electron beam.
A first object of the invention is a multiple beam device for the amplification of Rf power having a plurality of electron beam tunnels, each said tunnel carrying an electron beam formed by an electron gun. The multiple beam device consists of the following elements:
a plurality of drift tubes, the drift tubes separated to form a plurality of gaps associated with resonant cavities, including a first gap for the introduction of RF energy through an RF input port, and a final gap for the removal of RF energy through an RF output port,
an anode for the acceleration of electrons,
a magnetic field generator producing a radially symmetric field along a common axis defined by the beam tunnels,
and a plurality of magnetic field correctors for producing a magnetic field which is radially symmetric through each individual beam tunnel.
A second object of the invention is a multiple beam device having a plurality n of electron guns, each electron gun providing an electron beam traveling through an electron beam tunnel between a cathode and a beam collector, a common magnetic field applied to the beams of all n electron guns, individual magnetic field correctors applied to each individual gun, an RF input port, and an RF output port.
A third object of the invention is a multiple beam device having an input RF port and an output RF port common to all electron beamlets. xe2x80x9cA fourth object of the invention is a magnetic field correcter for an electron gun for a multi-beam klystron. A fifth object of the invention is a magnetic circuit comprising a magnetic field enclosure having a a central axis and end caps, a magnetic field generator inside this magnetic field enclosure, a plurality of beam tunnels located in the magnetic field enclosure, the beam tunnels coupled to a plurality of thermionic cathodes located external to the magnetic field enclosure and coupled to the beam tunnels through a plurality of apertures in the end caps, a magnetic field corrector to ensure the magnetic flux is perpendicular to the surface of the each cathode of the magnetic circuit, and an RF circuit which is coupled to the plurality of beam tunnels.
A sixth object of the invention is a magnetic field corrector comprising an end cap with plurality of apertures, each aperture comprising a first aperture for a beam tunnel and a second crescent aperture for magnetic field correction.xe2x80x9d